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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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424                                 ALASKAN GLACIER STUDIES
patently almost ceased in 1910 along the northern portion of line c and was just dying out near the lake shore. Melting had nearly healed the crevasses (PI. CLXIII), and the removal of the upper layers of ice by ablation had left a thin layer of morainie debris upon the ice in the easternmost zone.
If this explanation is correct, it naturally suggests the possibility of a similar explanation for the difference of moraine and vegetation in the middle and western zones, whose sharp contrast on either side of the line b-b calls for a special explanation. There is independent evidence that Miles Glacier had an earlier and greater period of expansion. This evidence comes from the southern margin of Miles Glacier where there is moraine and outwash with two independent sets of vegetation 20 and 70 years old respectively. It is, therefore, thought that during an advance 22 years or more before 1910 there was expansion of the northern bulb of Miles Glacier out to b. This resulted in crevassing and destruction of the vegetation which had presumably grown in the zone of thick ablation moraine, similar to the breaking of the ice and destruction of the vegetation which in 1906 took place in a part of the piedmont bulb of Atrevida Glacier. Certain faint ridges in the zone of thickest moraine suggest the former presence of crescentic cracks, similar to those that developed in the outer part of the forested zone on Atrevida Glacier, outside the zone of complete destruction of the morainie mantle and its vegetation. The rapid slumping, the presence of many kettles with pools, the youthful vegetation, and the lack of gradation features at the border (b-b), all accord well with this explanation. The difference between the bordering lines at b and c seems to be merely one of time. The uppermost portion of the middle zone still rises higher than the western zone, but at the border the relatively cleaner ice of the area of thick ablation moraine has melted down below the level of the very dirty, protected ice of the zone of thickest ablation moraine. In similar manner the west-facing cliff at c should, in time, be converted to an east-facing cliff, because of difference in rate of melting in the areas of thin and of thick moraine, unless continued advance in the zone of thin moraine makes up for the losses by ablation.
Western Terminal Moraine. On the western bank of Copper River, between Grin-nell Glacier and the mouth of Chinaman Slough, east of the railway bridge, there is a terminal moraine which seems to mark the westernmost extension of the Miles Glacier bulb.
Near Grinnell Glacier this terminal moraine forms a line of irregular hills extending nearly north and south between the railway and the northern part of Chinaman Slough (PI. CLXV). Mature cottonwood trees grow on this moraine, which is less than an eighth of a mile wide and SO to 50 feet high. Southward the moraine disappears beneath outwash gravels, reappears west of the railway and crosses the track north of Mile Post 50, and again disappears near the southern part of Chinaman Slough. But from here on to the site of the north ferry slip on the lake shore, its position may be traced practically all the way by the great numbers of large bowlders, some of which are from 10 to 12 feet in diameter. We believe that this belt of huge stones represents the residue of the terminal moraine after the river had washed away the clay, sand, and smaller bowlders. West of this moraine there are outwash gravels.
The Lake Basin. The site of the southern half of Miles Glacier bulb, east of the terminal moraine just described, is occupied by the lake basin, and the contiguous narrow zone of ground moraine from which the ice has completely disappeared. This